Inverse feedback circuits



Jan. 9, 1940. DALPAYRAT 2,186,195

INVERSE FEEDBACK CIRCUITS Filed Dec. 24, 1937 4 Sheets-Sheet 1 Hemni EDA LPAYRAT IN VEN TOR.

ATTORNEY Jan. 9, 1940. H. F. DALPAYRAT 2,186,195 V INVERSE FEEDBACK CIRCUITS Filed Dec. 24, 1957 4 Sheets-Sheet 2 w k N Q a m A T TORNEY 1940- v H. F. DALPAYRAT 2,186,195

nwanss FEEDBACK cIRcuI'l's Filed Dec. 24, 1937 4 Sheets-Sheet 5 HENRiF DALPAYRAT INVENTOR A TTORNE Y Jan. 9, 1940. F DALPAYRAT 2,186,195

' INVERSE FEEDBACK CIRCUITS Filed De c. 24, 1937 4 Sheets-Sheet 4 l l l l l l H mm P BA LPAYRAT INVENTOR' A TTORNEY Patented Jan. 9, 1940 P TI NT" o Fics I INVERSE FEEDBACK CIRCUITS Henri F. Dalpayrat, New York, N. Y., assignor to Radio Patents Corporation, New York, N. Y.-,;a corporation of New York Application December 24, 1937, Serial No. 181,696

7 Claims.

My invention relates to improvements in, and methods of operating amplifying circuits, for radio, telephone, telegraph, and other purposes, and. more particularly to inverse feedback systerns reacting upon high-pass and low-pass. filter circuits in audio frequency amplifiers.

The main object of my invention is to provide a novel method to suppress, limit, or expand, selected ranges of frequencies forming part of a signal, in order to correct distortions of the signal, and for altering the signal being amplified in such a way as to improve the quality of the reproduced signal, and improve the apparent selectivity of the tuning apparatus transmitting or receiving the signaling frequencies.

Inverse or negative feedback circuits as known in the art using discharge devices, such'as electronic relays, usually comprise a feedback arrangement whereby a portion of the amplified output .energy is fed back into the input circuit tolimit excessive signal amplitudes or suppress undesirable frequencies, resulting ina substan tial reduction of noises and distortion. 1

A disadvantage of inverse feedback circuits of this type known in the art, is the fact that while the excessive amplitudes are reduced, and the higher frequencies, usually composed of noises, aresuppressed, various parts of the desired signal are also reduced or suppressed, introducing I a new type of distortion and preventing the reproduction of a high-fidelity signal.

Accordingly it is an object of myinvention to provide a new type of inverse feedback circuit capable of altering in various ways the selected ranges of frequencies chosen through the adjustment of a simple controlling device.

Another disadvantage of inverse feedback cir-'- cuits, and particularly, inverse feedback circuits comprising frequency selective means such as bandpass filters, is the fact that the loss of energy in these filters is very high, necessitating high voltage variations in their input and consequently a larger number of filter sections, usually connected between different stages of amplification. This method is complicated, expensive and not reliable as the filters and all their connecting Wires must be carefully shielded to prevent interstage electromagnetic couplings and undesirable local oscillation noises and whistling noises which may result therefrom.

Accordingly it is a further object of my invention to decrease the number of filter sections to a minimum while obtaining a'high degree of efficiency without feeding back from one stage 1 back circuits, when designed for uses such as tone v control, a low or high-frequency boosteror suppressor, or asa means to obtain indirectly tun ing characteristics comparable to theideal flat top resonance curve Well known in the art, without decreasing the amplification of the tuned radio or intermediate frequency stages, as provided in a'radio receiving apparatus.

Still a further object of my inventionconsists in the provision of an efficient and simple circuit arrangement for manually adjusting the 25 quality and the selectivity of a received radio 1 broadcast signal as desired tosuit the personal .taste or requirements of the listener.

The invention hasfurther objects and aspe ctsin View which will appear hereafterin a detailed description thereof in connection with the accompanying drawings in which I have shown by I way of example several circuits embodying the novel features of the invention.

In the drawings, Figure 1 shows one embodiment of a translation system for electric wave energy comprising amplifying means with in- .verse feedback constructed according to themvention to obtain a desired frequency discrimination between an input and an output circuit, Figure 2 is a circuit showing a modification of the'system according to Figure 1, Figure 3 illustrates in block diagram form the previous circuit embodied in a radio receiving system, U "Figure 4 shows a further embodiment of the inventive circuit utilizing the screen grid of an amplifying valve for deriving inverse, feedback n y, j

Figure 51s a schematic diagram illustrating the use of inverse feedback derived from both the plate and screen grid of separate amplifying valves,

Figure-6 illustrates another modification of the e ionv e (o a nin frequenc d c m ati by means of filter networks associated with the inverse feedback paths.-

Similar reference characters identify similar parts throughout the difierent views of the drawings.

Figure 1 in the drawings shows terminals 1 and 2 receiving a signal and preferably a signal delivered by an amplifier comprising automatic volume control features, which are well known in the art, and need no special description here. The incoming signal causes voltage variations across two relatively high ohmic impedances .5 and 6 linking a filter I designed to transfer the high frequencies to another filter 8 designedto transfer the low frequencies.

The high frequencies are transferred by filter '5 through small condensers H and I3 while the lower frequencies leak to ground through resistance [2. Tube 9 thus amplifies mostly high frequencies of the incoming signal, and its efficiency for this purpose is further increased by a high cathode resistor l5 shunted by a small condenser I6. The amplified signal causes voltage variations in a resistor 24 connected in the anode circuit. The signal is delivered through a condenser 20. The tone or volume of the signal may be adjusted by varying a potentiometer resistance 2| by moving the contact ZIA. The respective amplifications of tubes 9 and it may be varied by increasing or decreasing the impedance of their output circuit resistors 24 and 3d through an adjustable contact 23. he low frequencies are preferably transferred to tube in through a high resistance 25 and a relatively large capacity 2'! while the higher frequencies leak to ground through a small capacity 25 and a low resistance 28. The efiiciency of tube if? is also further in.- creased for the amplification of lower frequencies through a cathode resistor 29 which is of lower value than that of cathode resistor i5. Resistor 29 is shunted by a relatively large condenser 30.

A part of the amplified energy of tube 9 is feedback through a de-coupling resistor fl and a condenser l8. Resistor i1 is needed to introduce damping in the feedback circuit and facilitate the adjustment of potentiometer 5 through contact I9. According to the setting of contact $9, the higher frequencies may be automatically reduced in intensity or if desired totally neutralized.

The operation of tube In is similar to that of tub-e 9 except that the input circuit of tube I0 is filtered to favor the transfer of lower frequencies as explained before. A portion of the amplified energy of tube It! is fedback through resistor 3i and condenser 32 and neutralizes in various amounts the low frequencies developed across resistor 6 according to the setting of potentiometer contact 33.

From this description it may be understood that either a given range of high frequencies. or a range of low frequencies may be reduced or neutralized. It is also evident that the reduction in intensity of all frequencies may be utilized to achieve a distortion-free volume control action. The reduction of certain frequencies provides an efficient tone control, while a sharp reductionof the higher frequencies, with a lesser reduction of the lower frequencies will substantially offset the effect of broadly tuned radio frequency circuits, thus causing a desirable improvement in the apparent selectivity of the tuned circuits.

While testing and perfecting this invention it has been found that resistors 24 and 34 could have fixed values, and be separate resistors instead of being branches of a potentiometer.

Potentiometer 2| and its sliding contact 2IA can also be eliminated and replaced by a conductor connecting condensers 20 and 35 to the output terminals YZ which may include a condenser 22.

When the contacts l9 and 33 are grounded to the cathode connection the complete system operates in a manner similar to two tubes connected in parallel amplifying equally all incoming frequencies, Displacing contact I9 along 5 reduces the intensity of the high frequencies, while displacing contact 33 along 6 reduces the intensity of the lower frequencies.

Figure 2 in the drawings, shows a circuit according to the invention connected between the output of an amplifier A. F. I and the input of another amplifier A. F. 2.

The circuit shown in Figure 2 is substantially the same as the circuit of Figure 1, but is further characterized by the inverse feedback circuits being connected to potentiometers '42 and 43 which form a part of the frequency selective filter networks, respectively.

The improvement in Figure 2 consists in the separated positions of the inverse feedback potentioineters which are easier to adjust than in Figure 1, and are capable of a finer degree of frequency selection due to their greater distances from the first input impedance of the filters.

Figure 3 shows the position of a circuit according to the invention in an amplifying system, such as a radio receiver where the signals received by an antenna (Ant) and ground, (Grd.) system are tuned and amplified by a tuned radio frequency system (T. R. F.) and demodulated by a detector (Det.) from which is also derived in a known manner a bias regulating voltage'for automatic volume control action or (A. V. 0.). The detected signal is amplified at audio frequency by an audio frequency amplifier (A. F. I) and then' passed through a circuit according to the invention or (A. F. S.)..

A part of the filtered and amplified energy is reduced or neutralized by inverse feedback (I. F. B.) only one feedback section being shown here for simplicity of illustration. final amplifier and (R) is the reproducer.

Although I have described my invention as a circuit utilizing two thermionic discharge devices it should be understood that a single discharge device can also be used provided that it contains the number of electrodes necessary for the connections to and the operation of my circuit.

I have also shown in my drawings, opposition feedback circuits connected to the anode output of the discharge devices, but I do not wish to be limited in this respect, as it is evident that for example, the opposition feedback circuits could be connected to the screen-grids while the amplified and desired currents could be obtained from the anodes, thus separating the opposition circuits from the output circuits, without departing from the spirit and scope of my invention.

An arrangement of this type is illustrated in Figure 4 wherein the inverse feedback circuits M and 45 are each connected to a positively charged grid in each electronic device.

The circuit in Figure 4 is similar in appearance and in operation to Figure 2, except that I have found that feedback from screen grids or positive grids had the advantage of efficiently audio frequency selector= (A. F. 2) is the ill reducing unwanted frequencies without decreasing appreciably the desired frequencies that. are amplified and delivered in the anode circuits, and without decreasing beyond practical limits the amplification of the electronic devices.

Figure 5 shows a combination of grid feedback and of anode feedback, which further assist in selecting various ranges of frequencies. An inverse feedback circuit 46 is connected between a phase into the input circuit through a common resistor 50, thereby reducing or neutralizing the ranges of frequencies selected by filters 48 and 49. While I have illustrated and described my invention'with specific reference to the embodiment according to the drawings it is obvious from the above that various modifications and variations may be made from the circuit as shown, coming within the broader spirit and scope of the invention, as pointed out in the following claims.

I claim:

1. An electrical circuit comprising two dis charge devices having their input circuits linked together by a common input impedance, each input circuit being filtered for difierent ranges of frequencies, and inverse feedback means in the output of each discharge device reacting upon the common input impedance to reduce the intensity of various ranges of signaling frequencies as selected by the filters connected in the input circuits of both discharge devices.

2. An electric translation system comprising an input circuit, a plurality of amplifiers, separate filter networks having difierent frequency .response characteristics connected between said input circuit and each of said amplifiers, a .common output circuit for said amplifiers, and adjustable inverse feedback means between points at a higher and lower oscillation level in at least one of said amplifiers.

3. An electric translation system comprising an input circuit, a plurality of amplifiers, separate filter networks having different frequency response characteristics connected between said input circuit and each of said amplifiers, a common output circuit for said amplifiers, and individual adjustable inverse feedback means between points at a higher and'lower oscillation level associated with each of said amplifiers.

' 4. An electric translation system comprising an input and an output, a plurality of parallel amplifying channels connected between said input and said output, electric filter means effective in passing predetermined partial ranges of component frequencies of electric wave energy to be transferred from said input to said output arranged in each of said channels, and adjustable inverse feedback means between points at a higher and lower oscillation level operatively associated with at least one of said amplifying ponent frequencies of electric wave energy to be, I

transmitted from said input to said output, and individual adjustable inverse feedback means between points at a higher and lower oscillation level operatively associated with each of said amplification channels.

6. A system as claimed in claim 4, wherein said channels comprise at least one amplifying valve nected between points at higher and lower oscillation level in each of saidamplified channels, electric filter means connected in each feedback path, said filter means being effective in passing predetermined partial ranges of component frequencies of electric wave energy to be transferred from said input to said output.

HENRI F. DALPAYRAT. 50 

